The presently disclosed embodiments are directed toward correcting positional distortion in an image of an object. Embodiments will be described with references to images of pages of bound books that are generated photographically or through the use of an image scanner. However, embodiments will find use in other image distortion and image manipulation applications.
With reference to FIG. 1, often, when a curved surface is imaged, a spatial or positional distortion of information in that image can occur. For example, in an image 104 of an opened book 108, the transformation from three-dimensional object space to two-dimensional image space causes a distortion. For example, portions of the object that were horizontal can appear to be curved upward or downward. For instance, a top edge 112 of a first page of the book 108 can appear to be curved downward in terms of the two-dimensional image 104. A bottom edge 114 can appear to curve upward. Similar distortions can be seen in other features of the book object. For example, lines of text in an upper portion 118 of the page also exhibit a downward curvature. Lines of text in a lower portion 122 of the page exhibit an upward curvature.
Such distortions can be inconvenient. In some instances, they make information on the object difficult to perceive. For example, text distorted in this way can be difficult for a human observer to read. Moreover, the accuracy and speed of optical character recognition software can be adversely affected by such distortions.
Accordingly, efforts have been made to develop mechanisms for correcting or compensating for such distortions.
For example, U.S. Pat. No. 5,497,236 by Wolff et al. discusses a method and apparatus for correcting for splay. According to Wolff, a document distorted by the curvature of a page of text away from a platen is converted to a digital image. The digital image is manipulated to remove the distortion by fitting lines of text in an unsplayed portion to a skew line, which represents the deviation of lines of text in the digital image from horizontal. Then the splay is determined for each line of text. Wolff then searches for a break point, which is that point in the image corresponding to the point on paper where the page begins to separate from the scanning platen. In embodiments of Wolff, a break point is found by independently evaluating each line and finding the point where an edge line separates from a skew line. This break point is used to identify the splayed portion of each edge line. The splayed portion of each edge line is fit to an exponential curve.
The processing of Wolf includes a number of additional procedures. The individual curve fitting of exponentials to each splayed portion is relatively time consuming. The situation is further aggravated with each additional splayed region (e.g., from a facing page or outer edge of a curled or curved document).
U.S. Pat. No. 7,170,644 B2 to Loce et al., entitled “Method and System for Reducing Distortion in Scanned Images,” discloses methods and systems for de-warping images that include image processing using an optics model and an illumination model which works well when the image is relatively uniform, such as a page of text at a constant point size and color.
The need remains for systems and methods to compensate for object curvature that are fast and are applicable to a broad class of images.